There are known in the prior art flotation dryers comprising a plurality of nozzles so arranged as floatingly to support the web being dried in a sine wave configuration. Such nozzles typically require discharge velocities in the order of 4,000 ft. per minute to provide a sufficient cushion to maintain good flotation of the web in a sine wave configuration.
While flotation dryers of the type known in the prior art are generally satisfactory for most webs, the velocities employed therein may cause flutter of very light gauge films or webs, thus creating the possibility of damage to the web or improper drying thereof. In addition, where heavy adhesive coatings, for example, are involved, velocities in flotation dryers of the prior art can cause problems by premature drying of the surface of the coating, thus inhibiting evaporation of solvents from within the coating, resulting in blistering and bubbling and the like.
In an attempt to solve the problem outlined above, there has been developed a flotation dryer nozzle in which a pair of outer slots form nozzles for directing streams of air at an angle toward each other to form a triangle when the nozzle is viewed in section. A perforated plate disposed between the slots permits air to flow from within the nozzle to the interior of the triangle so that the web is supported on the triangle. If the web is heavy, it sinks down into the triangle and starts penetrating the top thereof so that there is now a broader area of support for the web. At some stable equilibrium point the web will float, depending on tension and on the weight of the web. The problem with this proposed solution is that the velocity of air inside the triangle is such that it provides a scrubbing action which may cause overdrying or ripples in the web.
As an alternative to the arrangement in which auxiliary air was permitted to flow outwardly through perforations in a plate disposed between the nozzles, it has been suggested that air be permitted to flow inwardly through the perforations in such a plate and be exhausted to the atmosphere. We have discovered that this does not provide a solution to the problem since no appreciable inflow of air results.